1. Field of the Invention
This invention relates to a phenolic resin composition capable of being thermally cured and having both a long pot life and a fast curing rate. The present invention particularly relates to a phenolic resole resin composition and especially a phenol-formaldehyde resole resin composition containing a mixture of (a) a phenol-formaldehyde resole resin, and (b) a latent curing agent selected from a particular class of nitrogen-containing phosphorous compounds made from either phosphoric acid or phosphorous acid. The present invention is further directed to a method of making the phenolic resole resin composition, to a method of curing the phenolic resin composition and also to a method of making molded articles and composites using the phenolic resole resin composition.
2. Description of Related Art
Phenolic resole resins have long been cured with strong acids, such as sulfuric acid and organo-sulfonic acids. When used alone, these acids cause rapid hardening of most resole resins even at an ambient temperature, complicating the use of such resins for many applications particularly in making fiber reinforced plastic (FRP) and other resin composites and molded resin articles. The prior art, therefore, has long sought ways to retard the rate of resin cure at temperatures below the desired curing condition as a way of prolonging the pot life of such resin systems. To be useful, however, such cure rate retardation must be accomplished without degrading, to an unacceptable extent, the ultimate cure rate of the resin at the elevated cure temperature.
U.S. Pat. No. 5,243,015 represents one approach to solving this problem which uses a latent catalyst comprising a salt of a primary or secondary amine and a strong acid. This latent catalyst improves storage stability (pot life) of the resole resin composition, while providing a cure rate at an elevated temperature comparable to the rate obtained using conventional strong acid catalysts. Strong acids also may be added to the composition, to an extent to act synergistically with the latent catalyst to accelerate the rate of reaction; but preferably not in an amount which would cure the resin at a commercially useful rate by itself.
U.S. Pat. No. 5,378,793 (European Patent Application 539,098) represents another approach. This patent discloses using a partial phosphate ester as a hardener for phenolic resole resins. The partial phosphate ester is prepared by reacting (partially esterifying) condensed phosphoric acids, such as orthophosphoric acid, pyrophosphoric acid, tetraphosphoric acid or phosphorus pentoxide under relatively stringent conditions of temperature and vacuum, with polyols, such as glycerol, pentaertrythritol, sucrose and others to a constant free acidity value.
It is reported that the partial phosphate ester hardener exhibits a delayed action, i.e., the hardener cures the resole relatively slowly at a lower temperature, but allows a rapid cure when the temperature is increased. This presumably is due to dissociation of the ester at an elevated temperature to release free acid which acts as the hardener and promotes the cure reaction. Thus, it is the inherent acidity of the phosphate ester that causes the resole resin to cure. At lower temperatures, however, the partial ester is stable (i.e., has a relatively low acid value) and does not contribute to a premature cure of the resole.
There remains a need for a phenolic resin composition having a cure behavior that provides a long pot life at temperatures below curing conditions, but exhibits a rapid hardening at elevated cure temperatures.